1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines, which performs an abnormality diagnosis of an evaporative emission control system of the engine for purging evaporative fuel generated in the fuel tank into the intake system.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines, which comprises a fuel tank, a canister for adsorbing evaporative fuel generated in the fuel tank, and a purge control valve arranged across a passage extending from the canister to the intake pipe of the engine, for controlling purging of the evaporative fuel into the intake pipe.
An evaporative fuel-processing system of this kind has been proposed by U.S. Pat. No. 5,146,902, which has a function of performing a leakage diagnosis of an evaporative emission control system of the engine, by pressurizing the interior of the fuel tank by charging air thereinto by means of an air pump, detecting a rate of variation in the pressure within the fuel tank occurring after the pressurization, and determining that a leak has occurred from the evaporative emission control system when the rate of variation is large.
However, according to the above proposed leakage diagnosis method, there is a fear that a misjudgment is made depending on the condition of generation of evaporative fuel within the fuel tank. More specifically, when the interior of the fuel tank is pressurized to detect the rate of variation in the pressure within the fuel tank, in a condition where the ambient temperature is so high as cause generation of a large amount of evaporative fuel from the fuel tank, the tank internal pressure has already increased due to the generation of a large amount of evaporative fuel. As a result, the rate of variation in the pressure does not become large even if a leak has occurred, whereby a misjudgment can be made that the evaporative fuel emission control system is in a normal state.